1. Field of the Invention
The present invention relates to an attenuator controlling circuit provided in radio equipment such as a receiver or a transmitter-receiver, in which a time division multiple access (TDMA) system is adopted.
2. Description of the Related Art
In a TDMA system, a base station performs transmitting and receiving operations to and from a mobile station in accordance with a transmit-receive frequency assigned to the mobile station, and with a receiving slot and a transmitting slot within a frame. In this case, the receiving electric field varies according to the distance between the base station and the mobile station. For this reason, in order to prevent saturation of the receiving electric field and obtain the maximum output for a base band processing section, an attenuator is controlled such that the receiving power thereof is kept constant so as to cope with a change in the receiving electric field.
FIG. 2 is a functional block diagram of a conventional receiver in a base station.
In the receiver shown in this drawing, a signal inputted from an antenna 1 is amplified by a power amplifier 2 at a fixed gain, and the output is attenuated in a variable attenuator 3 in accordance with an output voltage of a comparator 12 which is considered to be a control signal.
Frequency conversion is carried out by a mixer (which will be hereinafter referred to as MIX) 4 and a local oscillator 5. The input signal is converted to an intermediate frequency lower than that of the input signal, and the frequency band is limited by a band pass filter (which will be hereinafter referred to as BPF) 6. Frequency conversion is further carried out by a local oscillator 8 and MIX 7, and the frequency band is further limited by BPF 9.
The thus obtained signal is amplified in a limit amplifier (which will be hereinafter referred to as LIM amplifier) 10 at a fixed gain up to an amplitude in which base band processing is allowed, and a modulation envelope is detected at a detector 11. A differential voltage between an output of the detector 11 and a reference voltage V.sub.ref, which indicates power in which LIM amplifier 10 is not saturated, is obtained in the comparator 12. The gain of the received signal is attenuated by the variable attenuator 3 with the thus obtained differential voltage as the control voltage. Through the above-described process, the received signal is controlled in a fixed receiving electric field.
However, the conventional receiver has the following drawbacks.
When the signal is controlled in the fixed receiving electric field of the receiver as described above, there exists a drawback in that a modulated signal such as a .pi./4-shift QPSK signal, which has information in its amplitude, loses amplitude information and is thereby not demodulated correctly.
In order to overcome the above drawback, there has been adopted a method in which an output of the detector 11 is integrated by a time constant circuit, and a timing for synchronization with a TDMA frame is controlled by a timing signal generating means in a control section so as to control a gain to be attenuated in the variable attenuator 3. However, in this method, when there is no burst wave, the gain to be attenuated in the variable attenuator 3 is 0 dB. For this reason, when the burst wave is thereafter inputted, an input modulated signal is greatly amplified. When the receiving electric field of the burst wave is large, saturation is caused. In addition, since integration of the output of the detector 11 is effected by the time constant circuit, more time for integration is required. As a result, there exists a drawback in that an AGC (Automatic Gain Control) loop cannot respond, and a head portion of the input signal is thereby distorted.